Anode production



Paiented Oct. 6, 1942 ANODE PRODUCTION Richard 0. Hull, Lakewood, Ohio,assignor to E. I. du Pont de Nemours & Company, Wilmington, Del., acorporation of Delaware No Drawing.

Application July 9, 1940,

Serial No. 344,498

Claims.

This invention relates to anodes, and more particularly to nickel anodesand processes for producing them in which electrolytically depositednickel is heat-treated and then quenched in suifuric acid.

When anodes of electrolytic cathode nickel are employed in the usualnickel plating solutions, the rate of corrosion of such anodes isordinarily slower than the rate at which nickel is removed from thesolution, so that the dissolved nickel content of the plating solutionbecomes depleted. In Hogaboom U. S. Patent 1,433,618 there is describeda process for heat-treating such nickel anodes to improve theircorrosion in nickel electroplating baths. Following the heat treatment,the nickel is cooled by air cooling or quenching.

Unfortunately, quenching of heated nickel in the usual media or by aircooling results in the formation of a passive nickel oxide on thesurface of the nickel, and this nickel oxide coating is exceptionallyresistant to attack upon being made anodic in the usual nickel platingsolutions. The oxide film must therefore be removed, and this is done byprocedures commonly known as deskinning." Such deskinning" operations,being an extra step in the manufacture of nickel anodes, add appreciablyto the cost of the anodes, besides being wasteful of the nickel.

The media heretofore available for quenching nickel have, moreover, notbeen entirely suitable on account of certain of their physical andchemical properties. Thus, when water is the medium used for quenching,a portion of the water coming into immediate contact with the nickel isconverted to steam, and this steam insulates 'the nickel momentarilyagainst further contact'with the water, and thereby r tards the rate ofheat transfer to the water. This insulating action is not uniform;hence, the grain structure of the quenched anode becomes irregular. Ifall is used as a quenching medium, the heat treated anodes must first becooled below the ignition temperature of the oil, as for instance by aircooling, and the relatively slow rate at which such preliminary coolingoccurs tends to permit resumption of the crystal structure which thenickel had before the heat treatment. Moreover, the 011 must be removedfrom the quenched anodes before they are used in nickel platingsolutions, and such removal entails still another step in the operation.Other quenching media heretofore used suffer from disadvantages similarto those just described for water or for oil.

Now I have found that nickel anodes having improved corrosioncharacteristics in electroplating solutions may be produced byheat-treating electrolytic cathode nickel and then quenching the nickelin a medium comprising concentrated sulfuric acid., Anodes so producedrequire no deskinning treatment to make them suitable for use. Moreover,they have a uniform and large grain structure and are characterized byeven, rapid corrosion during use in nickel plating solutions.

The nickel which may be treated according to the processes of myinvention preferably may be in the physical form in which it customarilyoccurs when produced as a cathodic deposit from electrolytic refiningbaths, running up to onehalf inch or more in thickness. It may besuitably supported as, for instance, on a sheet of nickel supported on astrap for making contact with a source of electric current and forsuspending the nickel in the plating solution, or it may be supporteddirectly on such a strap. Alternatively, the nickel may be in the formof cast electrodes prepared by melting cathode nickel and casting it toany desired shape, but such casting entails an extra operation andordinarily there is no advantage to be gained therefrom.

If desired,'there may be included in the cathode nickel small amounts ofother metals such as cobalt, cadmium or zinc. The amount of suchadditional metals used will ordinarily be relatively small, forinstance, less than about one per cent.

To heat treat nickel preparatory to quenching it in sulfuric acidaccording to a process of my invention, the nickel is heated by suitablemeans to a temperature at which crystal transformation will occur. Atemperature in the range from about 500 to about 1000" C. is ordinarilysuitable, a temperature of about 800 C. being usually preferred. Therate of heating and the time at which the maximum temperature ismaintained should be sufficient to permit crystal transformation andwill depend upon such factors as the thickness of the nickel and thetemperature attained. Once the nickel has been heated through, a fewminutes will ordinarily suffice to permit crystal transformation.

To quench heated nickel for anodes in accordance with a process of myinvention, the nickel is introduced into a body of concentrated sulfuricacid. Preferably the body of sulfuric acid is contained in a tank orother suitable receptacle of such size that the nickel may be completelysubmerged in the acid. It is desirable that the amount of acid berelatively large as compared with the amount of nickel in order thatquenching may be rapid and that the temperature of the acid will not beraised to an excessive degree. It will be apparent that discretionshould be exercised in introducing the heated" nickel into the acid toavoid raising the temperature of the acid suddenly above its boilingpoint. The conditions within the sulfuric acid quenching bath should besuch as will cool the nickel rapidly to a temperature below its criticalrange, that is, a temperature below that at which crystal transformationoccurs.

For best results the sulfuric acid used for quenching should berelatively concentrated. Ordinarily it is preferable that the quenchingbath contain no less than about seventy-three per cent by weight ofH2804, while an acid containing from about ninety-two to about onehundred per cent by weight of H2804 is'even more to be preferred. Ascompared with other quench.

ing media, sulfuric acid in these concentrations has a relatively highheat capacity and hence is particularly well adapted for the purpose.

The temperature of the acid used should preferably be held at a pointsubstantially below its boiling point. This may be done by using largeamounts of acid relative to the amount of nickel quenched, or by coolingthe acid by a suitable means such as cooling coils. Means forcirculating the acid in contact with the nickel to be cooled may also beprovided so as to obtain uniformity of temperature throughout the bodyof acid used.

In combination with the sulfuric acid employed, there may also be usedother acids suchas hydrochloric acid to improve further thecorrodibility of thequenched nickel in nickel plating baths.

The anodes producedby heat treating nickel and quenching it in sulfuricacid in accordance with a process of my invention are characterized byready solubility when made anodic in nickel plating baths andparticularly in such baths as those containing nickel sulfate in whichdissolution of nickel anodes has heretofore been a problem. The anodesare further characterized by a uniform and relatively large crystalgrain structure. The uniformity of grain structure causes the anodes tocorrode evenly when used in nickel plating baths, and such evenness ofcorrosion permits an increased proportion of the nickel present to serveits intended purpose of supplying nickel to the plating bath, therebeing a corresponding decrease in the amount of nickel lost due tosludge formation and corroding 01f of the anodes.

Nickel anodes produced according to the processes of my invention arefurther characterized by their freedom of a surface coating of oxide andby the fact that their nickel content is the same as the untreated anodefrom which they were produced, since the freedom from surface oxidemakes deskinning unnecessary. The surfaces of the anodes are relativelysmooth as compared with deskinned anodes because of the avoidance of adeskinning operation.

The foregoing description of my invention is further illustrated in thefollowing examples:

Example I Anodes fashioned from commercial electrolytically producedcathode nickel were heated to 800 C. for minutes in a muffle furnace.The anodes were then immediately submerged completely in commercialconcentrated sulfuric acid containing ninety-six per cent by weight ofH2804. When the anodes had cooled to approximately 6., they were removedfrom the acid, washed, and introduced as anodes in 9. Wattsnickel-plating solution having the following approximate composition:

' Oz./gal. solution Nickel sulfate, NlSO4'7H2O 32 Nickel chloride,NlClr-GHaO 6 Boric acid, H3303 4 The solution was then worked for anextended period at 120 F. and a current density of 35 amperes per sq.ft. of cathode, after which its nickel content was determinedanalytically. The nickel content was found to be substantially the sameas at the start of the period of use, indicating that the nickel of thesulfuric acid-quenched anode dissolved at a rate sufficiently rapid toavoid depletion of the nickel content of the bath. Examination of theanode showed that it had corroded evenly and that its surface wasrelatively smooth, the amount of sludge formation and metal loss due touneven corrosion being rel-- atively small as compared with the usualamount of sludge formation and metal loss occurring under similarconditions with anodes heat treated by processes heretofore used.

e Example I] Anodes prepared by electrolytically depositing nickelupon asheet of pure nickel suitably supported by straps to a total thicknessof one-half .inch were heated to 800 C. for thirty minutes in a mufilefurnace. The anodes were then immediately submerged completely in amedium comprising ten per cent by volume of commercial 20 B. muriaticacid and ninety per cent by volume of sulfuric acid containingninety-six per cent by weight H2804. When the anodes had cooled toapproximately C. they were remoi'ed from the acid, washed and furthercooled with water and introduced as anodes in a Watts nickel platingsolution having approximately the composltion shown in Example I. Thesolution was then used for an extended period for depositing nickel upona cathode, a temperature of F. and a current density of 35 amperes persquare foot of cathode being maintained. By analytical methods thenickel contents of the bath were both found to rise slowly during use.

It will be observed that the rate of corrosion of the anode preparedaccording to Example II was such as to permit an increase in the nickelcontent of the plating solution in which the anodes were used. This is,of course, a desirable characteristic since such an increase will offsetnickel losses due to dragout when such nickel plating solutions are usedin commercial operations.

While I have shown certain illustrative anodes and processs, it will beunderstood that one skilled in the art may without departing from thespirit l of this invention devise numerous processes and producenumerous anodes.

3. In a process for producing anodes from nickel, the steps comprisingheating the nickel to about 800 C., and th :1 quenching it to atemperature below its criti less than about seventy-three per centH2804.

4. In a process for producing anodes from electrolytically depositedcathode nickel; the steps 1 1 temperature in a sulfuric acid solutioncontaining not substantially 5 5. In a process for producing anodes fromelectrolytically deposited cathode nickel, the steps comprising heatingthe nickel to about 800 C., and then immediately quenching it to atemperature below its critical temperature range by immer i g itcompletely in a bath comprising about ten per cent by volume of 20 B.muriatic acid solution and ninety per cent by volume or concentratedsulfuric acid containing about ninety- 1 sixper cent by weight of H3804.

RICHARD o. HULL.

